1. Field of the Invention
The present invention relates to gate valves disposed in transport lines for transporting fluids containing mainly particulate materials, at coal gasification plants where coal is gasified and used as fuel for gas turbines and other devices.
2. Description of Related Art
Conventionally, coal gasification plants have been drawing attention because they make efficient use of abundant coal reserves. These types of coal gasification plants have transport lines for transporting fluids containing particulate materials, such as coal used as a fuel for a gasification furnace or unburned compounds generated from at the gasification furnace, between each constituent device. Valves for shutting off and sealing the fluids are provided to ensure operational control and safety of the plants.
With valves operated in such a particulate material environment, particulate materials enter the valve box and accumulate therein, which possibly obstructs the operation of a valve unit. Accordingly, measures for preventing the particulate materials from entering the valve box are required.
In addition, in some cases, sealing characteristics at high temperature and high pressure and durability in high-frequency operations are required at the same time, depending on the operating conditions at the plant.
Because the lines used for transporting fluids containing particulate materials usually have a large valve bore, it is not easy to remove the valve from the system. Accordingly, it is desirable that the valves have a structure that allows parts to be easily changed while remaining installed during inspection or maintenance of the valve.
In the related art, a through-conduit gate valve disclosed, for example, in Japanese Unexamined Patent Application, Publication No. HEI 5-141548 has been proposed as a valve for particulate materials used for preventing the particulate materials from entering gaps in the valve box and accumulating therein. This type of valve has a secondary seal, outside a primary seal, that seals the channel and the valve box of the known through-conduit gate valve to prevent the particulate materials from entering the valve box.
A known gate valve in which a seal is subjected to lower wear by abrasive fluids is a slurry gate valve disclosed, for example, in Japanese Unexamined Patent Application, Publication No. HEI 5-196153.
This type of gate valve has multiple gate plates that sequentially open; first, it shuts off fluids with an upstream gate plate and then operates a downstream gate plate to prevent a sealing portion on the downstream side from being damaged while the gate plate is partially opened.
In addition, a known gate valve that is less complex and externally adjustable without disassembling the valve is disclosed in, for example, Japanese Unexamined Patent Application, Publication No. SHO-54-142625. FIG. 11 shows the structure of a gate valve 100 disclosed in Japanese Unexamined Patent Application, Publication No. SHO-54-142625.
With this gate valve, the front and rear of a valve element 105 that has an opening portion 103 and a closing portion 101 are flanked by a main valve seat 107 and a sub valve seat 109, and a seal around a channel 111 is provided in front of and behind the main valve seat 107 and the sub valve seat 109, respectively. It is constructed so that the main valve seat 107 is moved back and forth using a bolt 113 to maintain surface pressure on a contact surface by the effect of a wedge. The gate valve 100 is opened and closed by the valve element 105 moving upward and downward.
However, the gate valve disclosed in Japanese Unexamined Patent Application, Publication No. HEI-5-141548 has a complex double sealing structure, and a seal is disposed on a moving portion; therefore, there is a problem in that the seal has low durability and is easily damaged.
The gate valve disclosed in Japanese Unexamined Patent Application, Publication No. HEI-5-196153 cannot prevent fluids from entering a valve box in a partially opened state. Accordingly, when the valve is used for fluids containing particulate materials, there is a problem in that motion of the gate valve is prevented due to the particulate materials accumulating in the valve box.
The gate valve disclosed in Japanese Unexamined Patent Application, Publication No. SHO-54-142625, similar to Japanese Unexamined Patent Application, Publication No. HEI-5-196153, also cannot prevent a fluid from entering a valve box in a partially opened state; therefore, there is a problem in that the operation of the gate valve 100 is disturbed by the particulate materials accumulating in the valve box when the fluid contains particulate materials.
When the gate valve 100 is used at high temperatures, a contact surface pressure exerted on the valve element 105 by the main valve seat 107 is adjusted by screwing the bolt 113; therefore, it is not possible to provide a seal with an accurate gap size for absorbing thermal expansion of the valve box body 115 generated at high temperatures. In addition, when loosening the main valve seat 107, the main valve seat 107 does not follow the bolt 113 even when it is loosened; therefore, there is a problem in that the main valve seat 107 and the valve element 105 cannot be secured.
In addition, because the sealing gasket 117 is provided on a sliding surface of the valve element 105, particulate materials easily enter the sealing gasket 117 and adhere thereto. When sliding in this state, the sealing gasket 117 is liable to be damaged and has low durability. Accordingly, it is difficult to maintain the sealing function.
Furthermore, there is a problem in that a complicated procedure is required to disassemble the valve in inspection and maintenance, namely, removing a cover 119, pulling out the main valve seat 107, removing an upper flange 121, and taking out the valve element 105 and the sub valve seat 109.